How do you secure purlins to the roof truss?

If you are using treated lumber (pergola etc...) you need to use fasteners that are treated coated or stainless. The fasteners should be applied in pairs at each truss, top and bottom. I recommend screws, particularly in treated wood, that are twice the thickness of the purlin. ( If the purlin is 2 inches thick, use 4 inch fasteners with a minimum of 1 inch piercing into the rafters) If you live in a snowy climate, the fasteners may be subject to shearing loads that preclude the exclusive use of screws for decked or roofed spaces. In that case, you would use hot dipped ring shank nails and use screws on the ends, seams, and every third rafter, to eliminate warpage from pulling the nails out out of the rafters. If you are applying purlins over decking, mark the rafters and use a fastener that is sufficient to pierce 1inch minimum into the rafter. Once again, use dipped treated or stainless fasteners for treated lumber and it might be preferable to use them as insurance in any case. In all cases, it would be best to use local code as your guide and to make sure that your work meets building codes. Checking with the local code enforcement inspectors is a good idea and there are often conditions that require a more stringent standard than the usual, such as high wind or snow loads. Finally, if you are applying a roofing product, chances are that there is information to be had on their website and any forums for the product. It is usually a good practice to build to the highest standard, rather than to suffer the failure of your roof or project.